To improve an energy usage rate of a power supply, power factor correction needs to be performed for the power supply, especially on the output of an alternating current power supply. In a low-power uninterruptible power system (UPS), the current is relatively small and generally a single-phase input and single-phase output mode is used, a power factor correction circuit generally employs a single-boost circuit structure. In a large-power UPS, considering the factors such as ripple suppression, inductor design, selection of semiconductor components, and actual power distribution conditions, a three-phase input and three-phase output mode is generally used. In the large-power UPS, a power factor correction circuit of a dual-Boost circuit structure needs to be used to proactively perform power factor correction on each phase of circuit.
FIG. 1 is a schematic structural diagram of a conventional three-phase power factor correction circuit. As illustrated in FIG. 1, a conventional three-phase power factor correction circuit employs three independent dual-Boost circuit structures; for each phase of input voltages, the circuit in a dual-Boost circuit structure performs power factor correction; each Boost circuit structure is formed of a diode, an inductor, an insulated-gate bipolar transistor (IGBT), and a capacitor. Specifically, the circuit for performing power factor correction on the A-phase voltage in FIG. 1 includes a Boost circuit B1 and a Boost circuit B2. The Boost circuit B1 includes a series circuit formed of a diode D1, an inductor L1, a diode D21, and a capacitor C1, and an IGBT that is connected between an output terminal of the capacitor C1, the inductor L1, and the diode D21. The Boost circuit B2 is a circuit structure symmetrical to the Boost circuit B1. The diode current direction in one Boost circuit is just opposite to the diode current direction in the other Boost circuit. In this way, the circuit B1 can perform power factor correction on the forward input alternating current voltage, and the circuit B2 can perform power factor correction on the inverse input alternating current voltage. In the power factor correction on the A-phase voltage, when the A-phase voltage is positive, the diode D1 is turned on, and the A-phase voltage undergoes power factor correction performed by the Boost circuit B1; when the A-phase voltage is negative, D4 is turned on, and the A-phase voltage undergoes power factor correction performed by the Boost circuit B2. In this way, power factor correction can be proactively performed on the A-phase voltage using two Boost circuit branches. Likewise, the power factor correction on the B-phase voltage and the C-phase voltage is the same as that for the A-phase voltage. Evidently, each phase of input voltages undergoes power factor correction proactively performed by a corresponding pair of Boost circuits, which can improve the power factor of the power supply effectively and improve the energy usage rate of the power supply.
However, in the three-phase power factor correction circuit in the prior art, there are six Boost circuit branches in total. Because three phases of input voltages are all sine wave voltages, each Boost circuit branch works only in a half of its normal working time, and the usage rate of the Boost circuit branch is low, which results in that a lot of components are used in the power factor correction circuit, and increases the costs. Meanwhile, because each Boost circuit works intermittently, the current peak value of each Boost circuit branch is large, which is unfavorable to the design of the inductor in the Boost circuit branch.